This invention relates to an improved gate turn-off thyristor (hereinafter referred to as "GTO") capable of cutting off a large current quickly and having a low turn-off loss and a power convertor using the thyristors.
Turn-off of the GTO is effected by extracting an anode current to a gate electrode by applying a reverse bias to a gate. Positive holes, that are injected from a p-emitter layer while the anode current is being eliminated, concentrate on a central portion of an n-emitter layer which is spaced apart from the gate electrode. When an area of a conduction region is extremely small and a turn-off time is long in a GTO for controlling a large current, thermal breakdown of the device is likely to occur due to the current concentration on the conduction region. A scheme for solving this problem has earlier been proposed in JP-A-59-132665. This scheme included forming an n-type impurity region having a high impurity concentration in such a manner as to come into contact with a p-emitter layer opposing an n-emitter layer. According to this structure, the positive holes injected from the p-emitter layer into the high concentration n-type impurity region are reduced by re-coupling (i.e. recombination), so that injection efficiency from the high concentration n-type impurity region is lowered and injection of the positive holes into the central portion of the n-emitter layer can be reduced. In devices for controlling a large current, however, the number of the positive holes injected from the p-emitter layer under a steady ON state is sufficiently greater than the impurity concentration of the high concentration n-type impurity region, and the major portion of the positive holes injected into the high concentration n-type impurity region reach the n-emitter layer without re-coupling. Therefore, the problem described above cannot be solved reliably.
A p-n-i-p-n structure wherein an n-base layer comprises a first layer portion having a low impurity concentration and a second layer portion having a high impurity concentration has been employed in GTOs having a high withstand voltage so as to increase an ON voltage and to prevent the drop of an operation speed (JP-A-63-205954). To form the second layer portion in this case, a method of forming the second layer portion by the diffusion of an impurity from a main plane on an anode side and a method of forming it by epitaxial growth of a high impurity concentration layer on the main plane of the first layer portion are generally known.
Further, the use of an anode short-circuit structure in combination with the p-n-i-p-n structure has also been employed to accomplish GTOs having a higher withstand voltage and excellent turn-OFF performance (JP-A-63-186473).
Nonetheless, the prior art described above does not consider the recoupling of carriers in a high concentration impurity region formed inside an n-base layer under a steady ON state in GTOs for controlling a large current density, and is not free from the problem that the current concentration occurs in the GTOs for controlling a large current, and their breakdown voltage is low. In GTOs having a high withstand voltage, on the other hand, the increase of the thickness of the n-base layer is unavoidable, and damping of a tail current gets retarded. The breakdown voltage of the GTOs according to the prior art is at most 4.5 KV, and a controllable current is at most 3 KA. The capacity of a unit inverter using such GTOs is as small as 8.3 MVA, and a greater capacity is difficult to attain.